Copper Oxide

The history of the diode is a fun one as it’s rife with accidental discoveries, sometimes having to wait decades for a use for what was found. Two examples of that are our first two topics: thermionic emission and semiconductor diodes. So let’s dive in.

Vacuum Tubes/Thermionic Diodes

Our first accidental discovery was of thermionic emission, which many years later lead to the vacuum tube. Thermionic emission is basically heating a metal, or a coated metal, causing the emission of electrons from its surface.

Electroscope

In 1873 Frederick Guthrie had charged his electroscope positively and then brought a piece of white-hot metal near the electroscope’s terminal. The white-hot metal emitted electrons to the terminal, which of course neutralized the electroscope’s positive charge, causing the leafs to come together. A negatively charged electroscope can’t be discharged this way though, since the hot metal emits electrons only, i.e. negative charge. Thus the direction of electron flow was one-way and the earliest diode was born.

Thomas Edison independently discovered this effect in 1880 when trying to work out why the carbon-filaments in his light bulbs were often burning out at their positive-connected ends. In exploring the problem, he created a special evacuated bulb wherein he had a piece of metal connected to the positive end of the circuit and held near the filament. He found that an invisible current flowed from the filament to the metal. For this reason, thermionic emission is sometimes referred to as the Edison effect.

Thermionic diode. By Svjo [CC BY-SA 3.0], via Wikimedia CommonsBut it took until 1904 for the first practical use of the effect to appear. John Ambrose Fleming had actually consulted for the Edison Electric Light Company from 1881-1891 but was now working for the Marconi Wireless Telegraph Company. In 1901 the company demonstrated the first radio transmission across the Atlantic, the letter “S” in the form or three dots in Morse code. But there was so much difficulty in telling the received signal apart from the background noise, that the result was disputed (and still is). This made Fleming realize that a more sensitive detector than the coherer they’d been using was needed. And so in 1904 he tried an Edison effect bulb. It worked well, rectifying the high frequency oscillations and passing the signals on to a galvanometer. He filed for a patent and the Fleming valve, the two element vacuum tube or thermionic diode, came into being, heralding decades of technological developments in many subsequent types of vacuum tubes.

Vacuum tubes began to be replaced in power supplies in the 1940s by selenium diodes and in the 1960s by semiconductor diodes but are still used today in high power applications. There’s also been a resurgence in their use by audiophiles and recording studios. But that’s only the start of our history.

On Hackaday, we usually end up featuring projects using building blocks (components, platforms…) that can be bought on the market. We however don’t show many hacks that rely on basic physics principles like the one shown in the picture above.

In the video embedded below, [nylesteiner] explains that copper oxide can be formed when heating a copper wire using a propane flame. When two oxidized wires are placed in contact with each other, an electrical current is produced when one wire is heated much hotter than the other. The trade-off is that the created thermocouple generates a small voltage but a ‘high’ current. However, when you cascade 16 junctions in series you can generate enough voltage to light up an LED. Even though the complete system isn’t particularly efficient at converting heat into electricity, the overall result is still quite impressive in our opinion. We advise our readers to give a look at [nylesteiner]’s article and blog to discover his interesting adventures.

Reader [Andre] sent in a link which tells us all about this “cool” Copper Oxide Thermoelectric Generator. All you need is a bit of solid copper wire and a gas torch. Burn the wire so it gets a nice coating of oxide. From there, it is a matter of making the 2 sections of burned wire cross at a point and heat up only one of the wires. Whichever is hotter forms a cathode and whichever one is cooler is the anode.

Just one of these junctions is enough to produce a few hundred millivolts, but the author takes it a step further, well 16 steps further. He made a ring of these junctions in series, which is enough to light a bright blue LED. While the author notes that this thing is producing a considerable amount of voltage, its not producing much amperage. This could come in very handy in the future, like if you need some additional LED lighting for your camp stove.